44 TEXT-BOOK OF PHYSIOLOGY. 



metabolism. Every cell presents in its nutritive activities an epitome 

 v bfrne nutritive activities of the body as a whole. 



Physiologic Properties of Protoplasm. All living protoplasm 

 possesses properties which serve to distinguish and characterize it 

 viz., irritability, conductivity, and motility. 



Irritability^ or the power of reacting in a definite manner to some 

 form of external excitation, whether mechanic, chemic, or electric, 

 is a fundamental property of all living protoplasm. The character 

 and extent of the reaction will vary, and will depend both on the 

 nature of the protoplasm and the character and strength of the 

 stimulus. If the protoplasm be muscle, the response will be a con- 

 traction; if it be gland, the response will be secretion; if it be nerve, 

 the response will be a sensation or some other form of nerve activity. 



Conductivity, or the power of transmitting molecular disturbances 

 arising at one~point to all portions of the irritable material, is also 

 a characteristic feature of all protoplasm. This power, however, is 

 best developed in that form of protoplasm found in nerves, which 

 serves to transmit, with extreme rapidity, molecular disturbances 

 arising at the periphery to the brain, as well as in the reverse direction. 

 Muscle protoplasm also possesses the same power in a high degree. 



Motility, or the power of executing apparently spontaneous 

 movements, is exhibited by many forms of cell protoplasm. In 

 addition to the molecular movements which take place in certain 

 cells, other forms of movement are exhibited, more or less constantly, 

 by many cells in the animal body e. g., the waving of cilia, the 

 ameboid movements and migrations of white blood-corpuscles, the 

 activities of spermatozooids, the projection of pseudopodia, etc. 

 These movements, arising without any recognizable cause, are fre- 

 quently spoken of as spontaneous. Strictly speaking, however, all 

 protoplasmic movement is the resultant of natural causes, the true 

 nature of which is beyond the reach of present methods of investi- 

 gation. 



Reproduction. Cells reproduce themselves in the higher ani- 

 mals in two ways by direct division and by indirect division, or 

 karyokinesis. In the former the nucleus becomes constricted, and 

 divides without any special grouping of the nuclear elements. It is 

 probable that this occurs only in disintegrating cells, and never in 

 a physiologic multiplication. In division by karyokinesis (Fig. 3) 

 there is a progressive rearranging and definite grouping of the nucleus, 

 the result of which changes is the division of the centrosoma, the 

 chromatin, and the rest of the nucleus into two equal portions, which 

 form the nuclei. Following the division of the nuclei, the proto- 

 plasm divides. The process may be divided into three phases: 

 i. Prophase. The centrosoma, at first small and lying within the 

 nucleus, increases in size and moves into the protoplasm, where 



